Rapid Access of New Pyranocarbazole Derivatives Under Microwave Irradiation

Background: Pyranocarbazoles exhibit promising biological activities such as mosquitocidal, antimicrobial, anti-inflammatory and antioxidant. Pyranocarbazole derivatives have also been used in the field of medicinal chemistry as a pharmacophore for different therapeutic indications. Methods: New pyrano[3,2-c]carbazole derivatives have been synthesized by the reaction of carbazole Mannich bases with (E)-N-methyl-1-(methylthio)-2-nitroethenamine under microwave irradiation. This transformation presumably occurs via o-quinone methide-Michael addition-tautomerism-intramolecular O-cyclization-elimination sequence of reactions creating new one C-O bond and one C-C bond. The structures of obtained pyranocarbazoles were confirmed by NMR, IR and mass spectra. Results: Various substituted carbazole Mannich bases such as fluoro-, nitro-, methoxy-, and trimethoxy-derivatives reacted smoothly to give the corresponding pyrano[3,2-c]carbazole derivatives in good yields. The significant features of this reaction include catalyst-free, solvent free, no column chromatographic purification, short reaction time and good yields. Conclusion: We have developed a simple and efficient synthesis of novel pyrano[3,2-c]carazoles derivatives under microwave irradiation in good yields. It is noteworthy that this reaction results in the formation of two new bonds (one C-C and one C-O) in a single operation.